JP4707096B2 - Gas sensor and manufacturing method thereof - Google Patents

Description

  The present invention relates to a gas sensor used for combustion control of an internal combustion engine.

  2. Description of the Related Art Conventionally, an oxygen sensor is known as a gas sensor that is installed in an exhaust system of an internal combustion engine, detects oxygen concentration in exhaust gas, and is used for combustion control of the internal combustion engine. This oxygen sensor has a detection element having a detection portion on the distal end side, and a cylindrical housing body that holds the detection element so that at least the detection portion is exposed on the distal end side. A cylindrical protective cover is provided at the front end of the housing in order to protect the detection portion of the detection element where the exhaust gas or the like is exposed to the gas to be measured.

  It is known that this cylindrical protective cover is joined to the outer periphery of the front end portion of the housing body by laser welding (see Patent Documents 1, 2, and 3). Accordingly, the protective cover can be firmly fixed to the housing body without dropping from the housing body.

Japanese Patent Laid-Open No. 9-210952 JP-A-9-304332 JP 2004-138599 A

  By the way, in order to improve the responsiveness of the gas sensor, it is conceivable to make the capacity in the protective cover smaller than before. This reduces the capacity in the protective cover, thereby improving the replaceability of the gas to be measured in the protective cover, thereby improving the responsiveness of the gas sensor. As one means for reducing the capacity in the protective cover, a step portion is provided in the protective cover, and a small diameter portion that is smaller in diameter than the large diameter portion on the rear end side than the step portion is provided on the front end side. Is mentioned. Thereby, since a small diameter part becomes smaller diameter than before, the capacity | capacitance in a protective cover can be made small.

  However, the protective cover having a small diameter portion may drop off from the gas sensor. This is because when the gas sensor is used, the protective cover heated by exhaust gas etc. is affected by vibration from rough roads, etc., and the step part of the protective cover that is most stressed cannot withstand this stress and cracks occur. Because it does.

  The present invention has been made in view of such conventional problems, and improves the responsiveness of the gas sensor, prevents cracks at the stepped portion of the protective cover, and prevents the protective cover from falling off. The present invention provides a gas sensor capable of satisfying the requirements.

The present invention extends in the axial direction and has a detection element having a detection part on the front end side, and the detection element is inserted into the inside of the tube in a cylindrical shape so that at least the detection part is exposed to the front end side. In a gas sensor having a housing main body that holds an element, and a cylindrical outer cover that is exposed to the outside and has a joint portion that covers the detection portion and is joined to the outer periphery of the distal end portion of the housing main body by laser welding.
The outer cover includes a large-diameter portion located on the distal end side of the joint and the housing body, a small-diameter portion whose outer diameter and inner diameter are smaller than the large-diameter portion, and a large-diameter portion and a small-diameter portion. An angle formed between the outer surface of the step portion of the outer cover and a direction perpendicular to the axial direction when viewed in a cross section including the axis, When the radius of curvature of the arc connecting the small diameter portion of the outer cover and the stepped portion is R (unit: mm),
θ> -30R + 45
Der is,
Wherein located between the outer cover and the detection element, the detection further includes a cylindrical inner cover which covers the element, characterized Rukoto such is joined the outer cover and the inner cover by spot welding And

  In this way, the outer cover is provided with a stepped portion that tapers toward the distal end side with respect to the large-diameter portion positioned on the distal end side relative to the joint portion and the distal end of the housing body, and further, the large-diameter portion on the distal end side In addition, a small-diameter portion where the outer diameter and the inner diameter are smaller is provided. Thereby, the capacity | capacitance in an outer side cover can be made relatively small, the substitution property of the to-be-measured gas in an outer side cover can be improved, and the responsiveness of a gas sensor can be aimed at by it.

Then, when viewed in a cross section including the axis, the angle formed between the outer surface of the step portion of the outer cover and the direction perpendicular to the axial direction is θ (unit is °), and the small diameter portion and the step portion of the outer cover are When the radius of curvature of the arc to be connected is R (unit: mm), θ> −30R + 45. By satisfying this, it is possible to prevent cracking at the stepped portion and to prevent the outer cover from falling off at a small diameter portion. Specifically, when the radius of curvature R of the arc connecting the small-diameter portion and the step portion of the outer cover is small, it becomes a vertex, the most stress is applied to the vertex, and the crack starts from the vertex. On the other hand, the starting point is eliminated by using the arc shape, the stress in the arc can be dispersed, and the outer cover can be prevented from cracking. And this circular arc can be effectively disperse | distributed by enlarging as the angle with respect to the perpendicular direction of a step part becomes large. When θ ≦ −30R + 45, the above effect cannot be obtained, and a crack may occur at the stepped portion.

The stepped portion connecting the large-diameter portion and the small-diameter portion may be a tapered portion that tapers toward the distal end side, and the angle θ formed with the direction perpendicular to the axial direction is 0 °. Thus, the right-angle part which becomes a right angle with respect to a large diameter part and a small diameter part may be sufficient.
In addition, the “angle between the outer surface of the step portion of the outer cover and the direction perpendicular to the axial direction” is perpendicular to the straight line and the axial direction when the outer surface of the step portion of the outer cover has a straight portion. The angle made with the straight line. On the other hand, when the large-diameter portion and the step portion are arc-shaped, and when the small-diameter portion and the step portion are arc-shaped, the tangent line that touches each other arc with respect to the inflection point that can be formed on the step portion of the outer cover. Is the angle between the tangent line and the straight line perpendicular to the axial direction.

  Furthermore, it is preferable that the step portion is located between the front end of the housing and the front end of the detection element when viewed in the axial direction. By forming the step portion at a position corresponding to the position where the detection portion of the gas sensor element is disposed, the effect of reducing the capacity in the outer cover can be further obtained, and the responsiveness of the gas sensor is effectively improved. .

  By the way, the outer cover having a Vickers hardness of 250 HV or more can protect the detection element covered inside without being affected by the impact even when an impact from the outside is applied to the outer cover. However, since such an outer cover has high hardness, it is difficult to release stress, and cracks are likely to occur particularly at a step portion where stress is easily applied. Therefore, by adopting the angle and the radius of curvature of the step portion of the present invention for the outer cover having such hardness, it is possible to prevent cracks at the step portion and to prevent the outer cover from falling off at a small diameter portion. From the viewpoint of manufacturing, the outer cover preferably has a Vickers hardness of 400 HV or less.

  Furthermore, it is preferable to have a cylindrical inner cover that is positioned between the outer cover and the detection element and covers the detection element, and the thickness of the outer cover is larger than the thickness of the inner cover. A gas sensor having an inner cover that covers a detection element inside an outer cover is known for taking measures against moisture of the element. At this time, by making the outer cover relatively thick, an external impact can be further absorbed by the outer cover. On the other hand, the inner cover is relatively thin to improve the workability of the inner cover.

  Hereinafter, a gas sensor as an embodiment to which the present invention is applied will be described with reference to the drawings. In the present embodiment, a gas sensor (oxygen sensor) that is mounted on an exhaust pipe of an automobile (particularly on the downstream side of the catalyst or the like) and detects the concentration of oxygen in the exhaust gas will be described. FIG. 1 is a cross-sectional view showing the overall configuration of the gas sensor 1 of the present embodiment.

  As shown in FIG. 1, the gas sensor 1 includes a sensor element 2 having a bottomed cylindrical shape with a closed end, a ceramic heater 3 inserted into a bottomed hole 21 of the sensor element 2, and the sensor element 2 inside thereof. The metal shell 4 is held. In the present embodiment, among the directions along the axis of the sensor element 2 shown in FIG. 1, the side (closed side, lower side in the drawing) toward the tip exposed to the measurement target gas (exhaust gas) is used. In the following description, the “front end side” is referred to as the “rear end side” and the side facing the opposite direction (upper side in the drawing) is referred to.

  The sensor element 2 is made of a solid electrolyte body having oxygen ion conductivity mainly composed of partially stabilized zirconia in which yttria is solid-solved as a stabilizer, and has a detection unit 22 at the tip. An internal electrode layer 23 formed in a porous shape with Pt or a Pt alloy so as to cover almost the entire surface of the bottomed hole 21 in the detection unit 22, and an internal electrode layer 23 on the outer surface of the detection unit 22. The external electrode layer 24 is formed in a porous shape in the same manner as in FIG. Further, an engagement flange portion 25 that protrudes radially outward is provided at a substantially intermediate position in the axial direction of the sensor element 2. The sensor element 2 of the present embodiment corresponds to a “detection element” in the claims. On the other hand, the ceramic heater 3 is formed in a rod shape and includes a heat generating portion 31 having a heat generating resistor therein. When the ceramic heater 3 is energized through heater lead wires 19 and 22 described later, the heat generating portion 31 generates heat, and has a function of heating the sensor element 2 to activate the sensor element 2. Fulfill.

  The metal shell 4 has a screw part 41 for attaching the gas sensor 1 to the attachment part of the exhaust pipe, and a tool engaging part 42 to which an attachment tool is applied when the gas sensor 1 is attached to the attachment part of the exhaust pipe. The metal shell 4 includes an alumina support member 5 that supports the sensor element 2 from the front end side, a filler member 6 made of talc powder that is filled on the rear end side of the support member 5, and the filler member 6 at the rear end. An alumina sleeve 7 that presses from the side toward the tip side is configured to be housed inside. The metal shell 4 of the present embodiment corresponds to a “housing body” in the claims.

  The metal shell 4 is provided with a metal side stepped portion 43 projecting radially inward on the inner periphery of the front end side, and the support member 5 is locked to the metal side stepped portion 43 via the packing 8. Yes. The sensor element 2 is supported by the metal shell 4 when the engagement flange portion 25 is supported on the support member 5 via the packing 9. A filling member 6 is disposed between the inner surface of the metal shell 4 on the rear end side of the support member 5 and the outer surface of the sensor element 2, and a sleeve 7 and an annular ring 10 are further provided on the rear end side of the filling member 6. It arrange | positions in the state inserted sequentially coaxially. The metal fitting 4 is fixed to the metal shell 5 by caulking the metal-side rear end 44 of the metal shell 4 in the inner tip direction. Note that the gas sensor 1 has a structure in which the filling member 6 is compressed and filled through the sleeve 7 by crimping the metal-side rear end 44 of the metal shell 4, whereby the sensor element 2 has a cylindrical shape. The metallic shell 4 is held in an airtight manner.

  Moreover, the front-end | tip part of the outer cylinder member 11 which consists of SUS304L is joined to the rear end side inner side of the metal shell 4. FIG. The outer cylinder member 11 is formed with an outer cylinder stepped portion 111 at a substantially intermediate position in the axial direction. The outer cylinder stepped portion 111 is formed at the tip side of the outer cylinder stepped portion 111 as an outer cylinder tip side body portion 112, and the outer cylinder stepped portion is formed. The rear end side of the portion 111 is formed as the outer cylinder rear end side body portion 113. Among these, the outer cylinder rear end side body portion 113 has a first caulking portion 114 for holding a holding member 17 described later and a second caulking for fixing an elastic sealing member 13 described later in an airtight manner. A portion 115 is formed.

  The separator 12 disposed in the outer cylinder rear end side body portion 113 of the outer cylinder member 11 via the holding member 17 passes through the element lead wires 20 and 21 and the heater lead wires 19 and 22. The separator lead wire insertion hole 121 is formed so as to penetrate from the front end side to the rear end side. Further, the separator 12 is formed with a bottomed holding hole 122 opened in the tip end surface in the axial direction. The rear end portion of the ceramic heater 3 is inserted into the holding hole 122, and the rear end surface of the ceramic heater 3 abuts against the bottom surface of the holding hole 122, whereby the ceramic heater 3 is positioned in the axial direction with respect to the separator 12. . The separator 12 has a separator flange portion 123 extending outward in the circumferential direction.

  The elastic seal member 13 disposed inside the rear end of the outer cylinder member 11 is electrically connected to the two element lead wires 20 and 21 electrically connected to the sensor element 2 and the ceramic heater 3. Four lead wire insertion holes 131 for inserting the two heater lead wires 19 and 22 are formed so as to penetrate from the front end side to the rear end side. Furthermore, an air introduction hole 132 is formed in the approximate center of the elastic seal member 13 from the front end side toward the rear end side, and a filter 133 is disposed inside the air introduction hole 132.

  The element lead wires 20 and 21 and the heater lead wires 19 and 22 pass from the inside of the outer cylinder member 11 to the outside through the separator lead wire insertion hole 121 of the separator 12 and the lead wire insertion hole 131 of the elastic seal member 13. It is pulled out towards. These four lead wires 19, 20, 21, and 22 are externally connected to a connector (not shown). The external signals such as the ECU and the lead wires 19, 20, 21, and 22 are input / output via the connector.

  Further, although not shown in detail, each lead wire 19, 20, 21, 22 has a structure in which the conducting wire is covered with an insulating film made of resin, and the rear end side of the conducting wire is a connector terminal provided in the connector. Connected. The leading end side of the conducting wire of the element lead wire 20 is caulked with the rear end portion of the terminal fitting 14 that is externally fitted to the outer surface of the sensor element 2, and the leading end side of the conducting wire of the element lead wire 20 is It crimps with the rear-end part of the terminal metal fitting 14 press-fit with respect to the inner surface of the sensor element 2. FIG. Thus, the element lead wire 20 is electrically connected to the external electrode layer 23 of the sensor element 2, and the element lead wire 21 is electrically connected to the internal electrode layer 24. On the other hand, the leading ends of the lead wires 19 and 22 for the heater are respectively connected to a pair of heater terminal fittings joined to the heating resistor of the ceramic heater 3.

Next, the outer cover 15 which is a main part of the present invention will be described in detail.
As shown in FIG. 2, the joint portion 151 of the outer cover 15 is joined to the distal end portion 26 of the metal shell 4. The outer cover 15 is made of austenitic stainless steel and has a bottomed cylindrical shape that covers the periphery of the detection portion 22 of the gas sensor element 2 protruding from the metal shell 4. The outer cover 15 is formed with a large-diameter portion 152, a stepped portion 154, and a small-diameter portion 153 in this order from the joint portion 151 toward the distal end side. Among these, the small diameter portion 153 is continuously provided in the circumferential direction of the outer cover 15 of the gas introduction hole 155 for introducing gas into the outer cover 15. In this way, the outer cover is provided with a stepped portion that tapers toward the distal end side with respect to the large-diameter portion positioned on the distal end side relative to the joint portion and the distal end of the housing body, and further, the large-diameter portion on the distal end side Since the small diameter portion where the outer diameter and the inner diameter are smaller than each other is provided, the capacity in the outer cover 15 can be reduced, and the replaceability of the gas to be measured in the outer cover 15 is improved, thereby The responsiveness of the gas sensor 1 can be improved.

The step 154 of the outer cover 15 has an angle θ1 formed by the outer surface f of the step 154 of the outer cover 15 and the direction perpendicular to the axial direction (t1 in FIG. 2) is 30 °. Further, the radius of curvature R of the arc p1 connecting the small-diameter portion 153 and the step portion 154 of the outer cover 15 is 1 mm. The angle θ1 and the radius of curvature R satisfy θ> −30R + 45, prevent cracking at the stepped portion 154, and prevent the small-diameter portion 153 of the outer cover 15 from falling off the metal shell 4.

  Further, the step portion 154 of the outer cover 15 is located between the tip of the metal shell 4 and the tip of the sensor element 2. By forming the step portion 154 at a position corresponding to the position where the detection portion 22 of the gas sensor element 2 is disposed, the effect of reducing the capacity in the outer cover 15 can be further obtained. Responsiveness is improved.

  Further, the outer cover has a Vickers hardness of 350 HV. Since the outer cover 15 has a Vickers hardness of 250 HV or more, even if an external impact is applied to the outer cover, the detection element covered inside is protected without being affected by the impact. Such an outer cover has a high hardness, so it is difficult to release stress, and cracks are likely to occur particularly at the stepped portion where stress is easily applied. By adopting θ = 30 °) and a radius of curvature (specifically, R = 1 mm), it is possible to prevent cracks at the stepped portions and to prevent the outer cover from falling off at a small diameter portion.

  Further, as shown in FIG. 1, an inner cover 16 is formed inside the outer cover 15. Similar to the outer cover 15, the inner cover 16 has a bottomed cylindrical shape that covers the detection portion 22 of the gas sensor element 2, and is made of austenitic stainless steel. The inner cover 16 is also formed with a large-diameter portion 161, a stepped portion 163, and a small-diameter portion 162 in this order from the rear end side. Like the outer cover 15, the gas introducing hole 164 is formed in the circumferential direction in the small-diameter portion 162. It is connected continuously. The outer cover 15 is formed with a recess (not shown) that prevents the inner cover 16 from rotating, and the recess contacts the large-diameter portion 161 of the inner cover 16. Further, the large-diameter portion 161 is joined to the outer cover 15 by spot welding at four points around the small-diameter portion 153 of the outer cover 15.

  In the present embodiment, the outer cover 15 is thicker than the inner cover 16. Specifically, the thickness of the outer cover 15 is 0.5 mm, and the thickness of the inner cover is 0.3 mm. Thus, by making the thickness of the outer cover relatively thick, an external impact can be further absorbed by the outer cover.

  Next, the manufacturing method of the gas sensor 1 of this embodiment is demonstrated in detail. First, after adding 5 mol% of yttria to zirconia and granulating, it is formed into a bottomed cylindrical shape having a closed tip and fired at a temperature of 1400 to 1600 ° C. in an electric furnace to obtain a solid electrolyte body 22. It was. Next, the outer electrode layer 23 made of platinum is provided on the outer peripheral surface of the solid electrolyte body 22 by vapor deposition, chemical plating, or the like. On the other hand, the inner electrode layer 23 was similarly provided on the inner surface of the solid electrolyte body 22 using vapor deposition, chemical plating, or the like, and the gas sensor element 2 was obtained.

  Next, the joint portion 151 of the outer cover 15 in which the inner cover 16 is inserted is inserted into the distal end portion 26 of the metal shell 4 and is welded all around by laser welding. Then, packing 8, support member 5, packing 9, gas sensor element 2, filling member 6, and sleeve 7 are sequentially inserted into the metal shell 4, and the metal sensor side rear end 44 of the metal shell 4 is swaged to prepare a sensor lower intermediate body. To do.

  On the other hand, the element lead wires 20 and 21 are joined to the terminal fittings 14 and 14, respectively, and the heater lead wires 19 and 22 are joined to the heater terminal fittings of the ceramic heater 3. Then, the lead wires 19, 20, 21, 22 are inserted into the separator lead wire insertion holes 121 of the separator 12 with the ceramic heater 3 positioned inside the terminal fitting 14. Next, in a state where each lead wire 19, 20, 21, 22 is inserted through the lead wire insertion hole 131 of the elastic seal member 13 inserted into the outer cylinder member 11, the distal end surface of this elastic seal member 131 is the separator 12. Move until it touches the rear end face. And the holding member 17 is inserted from the front-end | tip of the separator 12, and the outer cylinder member 11 is crimped so that the holding member 17 may be fixed, and the 1st crimping part 114 is formed. In this way, the sensor upper intermediate is produced.

  Then, the outer cylinder member 11 of the sensor upper intermediate body is inserted into the rear end side of the metal shell 4, and the overlapping portion of the outer cylinder member 11 and the metal shell 4 and the elastic seal member 13 are respectively crimped. The elastic seal member 13 is formed with a second caulking portion 115. The caulking was performed by Happomaru caulking. The overlapping portion is fixed by laser welding to complete the gas sensor 1.

In order to confirm the effect of the present invention, the following various tests were performed.
In the test, only the outer shell 15 and the inner cover 16 of the metal shell 4 are used. First, the metal shell 4 made of SUS310 is manufactured in a predetermined shape, and then the joint 151 of the outer cover 15 in which the inner cover 16 is inserted is inserted into the tip portion 26 of the metal shell 4, and all-around welding is performed by laser welding. did. The outer cover 15 is made of SUS310, has a thickness of 0.5 mm, and further, the length from the tip of the outer cover 15 to the tip of the metal shell 4 is 16 mm. On the other hand, the inner cover 16 is made of SUS310 and has a wall thickness of 0.3 mm. Then, various samples were prepared in which the angle θ1 of the step portion 154 of the outer cover 15 and the radius of curvature R1 of the arc p1 were as shown in Table 1.

  And various samples were attached to the thermal shock apparatus 200 as shown, for example in FIG. Specifically, the sensor thermal shock device 200 includes a pedestal 201, a vibration part 202 attached to the pedestal, and a metal shell attachment part 203 attached to the vibration part 202 so as to connect the pedestal 201 and the vibration part 202. A spring 204 is attached. The movement of the sensor heating shock device 200 will be briefly described. The vibrating portion 202 vibrates (vertical direction in FIG. 3) due to expansion and contraction of the spring 204.

  Next, the burner 205 was applied from the front end side of the outer cover 15 and heated so that the front end temperature of the outer cover 15 was 1000 to 1100 ° C. Thereafter, the vibration unit 202 was vibrated for 15 minutes so that the amplitude width was 5 mm and the frequency was 6.7 Hz (400 rpm). Thereafter, the vibration of the vibration unit 202 and the burner 205 are stopped, and the outer cover is cooled to room temperature. The above process was set as one cycle, and after performing this three cycles, it was checked whether or not the step portion 154 of the outer cover was broken. The results are also shown in Table 1. In Table 1, what was broken at the step portion 154 was ×, what was broken at the joint portion 151 between the metal shell 4 and the outer cover 15, and what was not broken at the step portion 154 and the joint portion 151. And

  According to Table 1, when the angle θ1 was 15 °, it was “x” when R1 was 0 mm, 0.5 mm, and 1.0 mm, but it was “good” when the angle θ1 was 1.5 mm or more. In addition, when the angle θ1 was 30 °, the R1 was 0 mm and 0.5 mm, but it was “x”, but when the angle θ1 was 1.0 mm and 1.5 mm or more, it was “good”. When the angle θ1 was 45 °, it was “x” when R1 was 0 mm, but it was “◯” when 0.5 mm, 1.0 mm, and 1.5 mm or more. Further, when the angle θ1 was 60 °, all of R1 of 0 mm, 0.5 mm, 1.0 mm, and 1.5 mm or more were evaluated as ◯. That is, the angle θ1 and the radius of curvature R1 are θ> −30R + 45, so that cracks at the stepped portion 154 can be prevented, and the small-diameter portion 153 of the outer cover 15 can be prevented from falling off the metal shell 4.

The embodiment of the present invention has been described above. However, the present invention is not limited to the embodiment, and various design changes can be made within a range in which the object of the present invention can be achieved.
For example, in the present embodiment, the gas sensor 1 using the bottomed cylindrical gas sensor element 2 is taken as an example, but the present invention is not limited thereto, and the gas sensor 1 using the plate-like gas sensor element 2 may be used.
In this embodiment, the double cover using the outer cover 15 and the inner cover 16 is used. However, the present invention is not limited to this, and only the outer cover 15 may be used. (Three or more covers) may be used.

It is sectional drawing of the gas sensor 1 of this embodiment. It is a principal part expanded sectional view of the gas sensor 1 of this embodiment. It is explanatory drawing of the sensor heating impact apparatus 200 used for an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Gas sensor 2 ... Gas sensor element 3 ... Heater 4 ... Main metal fitting 15 ... Outer cover 16 ... Inner cover 151 ... Joint part 152 ... Large diameter part 153 ...・ Small diameter part 154... Step part 155... Gas introduction hole 200.

Claims (4)

A detection element extending in the axial direction and having a detection part on the tip side;
A housing main body that is cylindrical and that inserts the detection element inside itself, and holds the detection element so that at least the detection portion is exposed on the distal end side;
In the gas sensor having a cylindrical outer cover exposed to the outside, which covers the detection part and has a joint part which is joined to the outer periphery of the front end part of the housing main body by laser welding,
The outer cover includes a large-diameter portion located on the distal end side of the joint and the housing body, a small-diameter portion whose outer diameter and inner diameter are smaller than the large-diameter portion, and a large-diameter portion and a small-diameter portion. Including a step portion located between
When viewed in a cross-section including the axis, the angle formed between the outer surface of the step portion of the outer cover and the direction perpendicular to the axial direction is θ (unit: °), and the small-diameter portion of the outer cover and the step portion are connected. When the radius of curvature of the arc is R (unit: mm),
θ> -30R + 45
Der is,
A cylindrical inner cover that is located between the outer cover and the detection element and covers the detection element;
Gas sensor and the outer cover and said inner cover and said Rukoto such are joined by spot welding. The gas sensor according to claim 1, wherein
The gas sensor according to claim 1, wherein when viewed in the axial direction, the stepped portion is located between a front end of the housing and a front end of the detection element. The gas sensor according to claim 1 or 2,
The step of the outer cover has a Vickers hardness of 250 HV or more. The gas sensor according to any one of claims 1 to 3,
The gas sensor according to claim 1, wherein the outer cover is thicker than the inner cover.